There has been widely known a scanning electron microscope configured to generate and display a two-dimensional image of a scanned observation target region of a sample by irradiating the sample with a converged primary electron beam in the way of two-dimensionally scanning the observation target region on the surface of the sample with the primary electron beam, detecting signal electrons emitted from an irradiation position with the primary electron beam, and mapping the detection signal in synchronization with a scanning position of the primary electron beam.
The signal electrons emitted from the sample surface are classified broadly into secondary electrons and reflected electrons according to energy. The reflected electrons are electrons emitted from the sample surface again after incident electrons repeat elastic scattering and non-elastic scattering within the sample. Therefore, the reflected electrons have a peak in a generation volume at approximately the same energy level as the incident electrons. On the other hand, the secondary electrons mean electrons emitted from the sample surface among low-energy electrons generated from the reflected electrons when they undergo non-elastic scattering. Therefore, the secondary electrons have a peak in a generation volume at an energy of about several eV. Generally, signal electrons having an energy of less than 50 eV are called secondary electrons to be distinguished from the reflected electrons.
As for the scanning electron microscope, it has been known that an observed image (secondary electron image) of the sample based on the secondary electrons and an observed image (reflected electron image) of the sample based on the reflected electrons contain different information. In other words, the secondary electron image is an observed image highlighting information on unevenness and potentials on the sample surface, while the reflected electron image is an observed image highlighting information on sample composition and crystal orientation. This is because of the following reason. Specifically, in the case of the secondary electrons, the generation volume is likely to be influenced by the shape of the sample surface or surface potential due to the small energy. On the other hand, in the case of the reflected electrons, the generation volume depends on the average atomic number of the sample. Moreover, it is also known that channeling contrast also results from the reflected electrons, the channeling contrast being observed on a sample having the same composition in the case where the crystal orientation on the sample surface is partially different or a crystal defect or the like is contained.
The scanning electron microscope has been required to more clearly capture not only the unevenness on the sample surface but also the contrast or channeling contrast representing a sample composition as described above. In other words, for the scanning electron microscope, there has been a demand for a method of acquiring a sample observation image based on reflected electrons quickly and easily with high sensitivity.
Generally, in order to irradiate a primary electron beam onto a sample from above and detect reflected electrons, an electron beam detector needs to be disposed immediately above the sample. In such a case, because of the need to reduce the size of the electron beam detector, there has heretofore been used a single crystal scintillator as disclosed in Patent Document 1, for example, as such an electron beam detector.
Moreover, in order to obtain a reflected electron image with clearer contrast or channeling contrast that represents the sample composition, noise of the image needs to be reduced by detecting reflected electrons while separating them from secondary electrons as much as possible, for example. Patent Documents 2 to 4 disclose a method for separating reflected electrons from secondary electrons based on a difference in energy or trajectory.